Heat transfer limitations and optimization have been an engineering design constraint for decades. Thermal management has been a key consideration in the design and development of military hardware in the past century. More recently, cooling effectiveness has become a very important technical challenge and is one of the limiting factors in the further development of a range of military related disciplines including electronic, high-energy weapon and propulsion systems. Microelectronic components are particularly susceptible to thermal management problems and have become an integral component in most military systems. Many of these components cannot operate at elevated temperatures resulting in a thermal management system becoming a key consideration.
Convective heat transfer is one way of addressing thermal management. Convective heat transfer is the heat transfer process that is executed by the flow of a fluid over a surface of a medium. Convective heat transfer includes advective heat transfer, which is based on the velocity of the fluid flow compared to the medium, and conductive heat transfer, which is based on static fluid adjacent to the medium. In convective heat transfer, the fluid acts as a carrier for the energy that it draws from (or delivers to) the surface of the medium.
FIG. 8 illustrates a convective heat transfer system wherein a fluid 804 flows with a velocity f adjacent to a medium 802. When fluid 804 passes adjacent to medium 802, the portion of fluid 804 that is directly adjacent to medium 802, i.e., at the boundary of medium 802, has a velocity of zero as a result of a sheer stress created at medium 802. The velocity of fluid 804 increases to a maximum as the distance from the boundary of medium 802 increases. A layer of fluid 804 adjacent to the boundary of medium 802, referred to as the boundary layer 806, is the layer fluid having a velocity much lower than the average velocity of the remainder, or bulk, of the flowing fluid. As illustrated in FIG. 8, flowing fluid 804 creates a boundary layer 806 adjacent to the boundary of medium 802 such that only a bulk of fluid 804, or bulk fluid 808, essentially flows.
For purposes of a simplistic explanation of heat transfer in the system of FIG. 8, assume that medium 802 has a temperature Hm, whereas bulk fluid 808 has an average temperature Hf, wherein Hm≠Hf. The term “average” is used to describe the temperature Hf of bulk fluid 808 because bulk fluid 808 will have many different local fluid temperatures, but the overall temperature of bulk fluid 808 can be generally described using the average of such temperatures. If Hm<Hf, then heat will be convectively transferred from bulk fluid 808 to medium 802 through boundary layer 806. Alternatively, if Hm>Hf, then heat will be convectively transferred from medium 802 to bulk fluid 808 through boundary layer 806.
There are many ways to specify the types of convection. The flow over the surface can be specified as internal, e.g., with pipes or ducts, or external, e.g., with fins. The motive force behind the bulk fluid motion can be forced, e.g., by a fan or pump, or natural, e.g., driven by buoyancy forces caused by fluid density changes with temperature. The flow can be further classified as single-phase, wherein the fluid does not change phase or multi-phase, e.g., boiling or condensation.
There are many specific characteristics of the flow of a fluid that greatly affect the heat transfer rate from/to the medium's surface, but the two categories that govern the effectiveness of single-phase forced convective heat transfer are: 1) the rate of conduction of energy (heat) to/from the medium surface; and 2) the rate of conveyance of energy toward/away from the surface with the mass flow of the bulk fluid. The rate of conduction is dictated by both the thermal conductivity of the fluid and the temperature of the fluid in the boundary layer. The thermal conductivity of the fluid is a temperature dependent physical property of the fluid that is being used in the convection process. The temperature of the fluid in the boundary layer is influenced by the amount of heat transferred, the specific heat of the fluid and the flow characteristics in the boundary layer. Poor flow characteristics will not allow the fluid in the boundary layer to be replaced by the bulk fluid. The major factors that determine the rate of energy conveyance are the mass flow rate of the bulk fluid and the specific heat capacity of the fluid.
The best convective heat transfer occurs when the fluid properties and flow conditions are optimized. The optimal fluid properties are high thermal conductivity and high specific heat capacity. The flow conditions that favor optimal convective heat transfer include high local fluid velocity at the medium's surface. Unfortunately, it is difficult to optimize both the thermal conductivity and specific heat capacity of a fluid, and the naturally occurring boundary layer limits the flow near the medium's surface.
Two specific areas of convective heat transfer research address the fluid property and surface flow problems. These two areas include the use of nanofluids and the use of magnetic fields with magnetrohetrological fluids. Both have limited success in enhancing the rate of convective heat transfer.
Nanofluids are conventional fluids with tiny particles therein that may typically be no larger than several nanometers. The particles are usually of high thermal conductivity and are added to the fluid to increase the bulk thermal conductivity of the fluid. In general, the particles are metal or metal oxides, such as for example Cu, CuO and Al2O3. A significant increase in thermal conductivity has been reported for various volume fractions of particles suspended in different fluids. Experiments performed utilizing nanofluids have shown an increase of convective heat transfer rate when compared to the same fluid without nanoparticles.
The bulk majority of the research in magnetic fields used to enhance heat transfer is focused on the hydrodynamic manipulation of magnetorhetrological fluids (ferrofluids). Much of the numerical and theoretical investigation centers on a disruption of the boundary layer through the use of a constant magnetic field acting on a ferrofluid. In all of these cases the fluid is assumed to remain homogeneous in particle composition. Another area of research utilizes magnetic fields and soft magnetic particles to reduce the disadvantage of inefficient gas-solid two-phase flow. The magnetic particles are attracted to the wall, which has a temperature that is higher than the temperature of the bulk fluid flowing by the wall. The attracted particles are heated above their Curie point by thermal conduction and then are carried away by the flow. By conservation of energy, the temperature of the wall is generally decreased by an amount proportional to the amount of heat carried away, whereas the temperature of the bulk fluid is increased by an amount proportional to the amount of heat carried away.
Neither the use of nanofluids nor constant magnetic fields, described above, optimize the potential for improving convective heat transfer performance.
What is needed is system and method for improving convective heat transfer performance.